Tesla has confirmed that it has hit the 200,000th US delivery threshold. This milestone triggers the phase-out period.

Tesla buyers will lose the full $7,500 credit on new or inventory vehicles by the end of 2018, phasing to $3750 for the first half of 2019 and $1875 in the second half of 2019.

The big push for Model 3 production expedited this phase out milestone. However it appears Tesla intentionally timed the 200,000 vehicle to be sold in July (Q3) as to extend the phase out timeline.

For new and inventory Model S, X, and 3 buyers the federal phase out will look like this, under current rules.$7,500On or before December 31, 2018$3,750January 1 to June 30, 2019$1,875July 1 to December 31, 2019

There’s a new bill in Congress that is attempting to remove the delivery limit for the federal tax credit and replace it with a time limit but it doesn't seems likely to pass at this time.